1. Field of the Invention
The present invention relates to an ignition device and in particular to an ignition device for high frequency discharge, used in the operation of an internal combustion engine.
2. Description of the Related Art
In recent years, problems of environmental conservation or fuel depletion have been posed, and there is a pressing need even in the automobile industry for countermeasures for those problems. As one example of such countermeasures, there is a method of improving fuel consumption dramatically by down-sizing and reducing weight of engines using superchargers.
It has been known that in a highly supercharged state, the pressure in the combustion chamber of an engine becomes very high even in a state where combustion has not started and that a spark discharge for starting the combustion is difficult to initiate. One of the reasons is that the voltage required to cause an insulation breakdown at the gap between the high voltage side electrode and the ground side electrode of an ignition plug becomes very high, exceeding the breakdown voltage of the insulator portion of the ignition plug.
Although research for increasing the breakdown voltage of the insulator portion to solve this problem has been made, it is difficult actually to secure a sufficient breakdown voltage for the requirement and there is no other choice but to adopt means for narrowing the gap of the ignition plug.
However, narrowing the gap of the ignition plug will instead expand the quenching action at the electrode, causing another problem of reduction in start-up performance and combustion performance.
For solving this other problem, an ignition device provided with means to avoid such problem as by supplementing energy in excess of the quenching action, i.e. thermal energy absorbed by the electrode with a spark discharge, or for inducing combustion at a portion even just a little removed from the electrode has been proposed as described in e.g. Patent Document 1 (Japanese Patent Application Laid-open No. 2012-112310).
The ignition device disclosed in Patent Document 1 above enables the spark discharge to be initiated at the gap of the ignition plug by means of a conventional ignition coil and a high frequency current made to flow in the path of the spark discharge via a mixing portion including a capacitor, thereby forming a discharge plasma that is a spark discharge of high energy and that further extends over a wider area than a conventional spark discharge.